Most aquarium visitors are there to see sharks, sea turtles, fish and other marine life. But at the National Aquarium in Baltimore, there's another star attraction: Coral.

The Aquarium's Blacktip Reef exhibit will open later this summer, and give visitors a look at an Indo-Pacific coral reef. But curators can't just carve a chunk out of a wild reef to put in the vast tank, that would destroy the very ecosystem for which they hope to raise conservation awareness. And corals take hundreds of years to develop into a reef, so the aquarium can't grow its own in-house.

Which leaves only one option: Somebody has to make a fake reef, one that is realistic enough to convince visitors and marine life. At the National Aquarium, that task falls to Paul Valiquette, director of Fabrication in the Exhibits and Design Department, and his team of sculptors and scientists working out of a warehouse down the street from the aquarium.

"The coral rock is made by creating a form, basically a big fiberglass shell, and on that goes texture — gravel and sand — and it gets base painted with epoxy paint," says Valiquette. All the materials have to be durable enough to withstand years in corrosive salt water, but be flexible enough that they won't break like real coral would.

"Of course, nobody really makes stuff for making coral reefs, so a lot of it is cross technologies," Valiquette explains as he removes a rubber Stag-horn coral branch from a mold.

Back at the National Aquarium exhibit, the fake coral is arranged to form a comprehensive reef structure. The underwater viewing windows look out on three different environments: an underwater coral cave, an open reef and dense flats of small branching corals that a reef explorer would find closer to shore.

In the aquarium, everyone can see this fragile reef ecosystem — and it's shark and fish inhabitants — in a way that preserves the real Indo-Pacific reefs and the corals that create them.

Today is World Oceans Day, a perfect excuse for a visit to the National Aquarium in Baltimore, Maryland, where they're prepping for the grand opening of a new $13 million wing.

In early July, the Blacktip Reef exhibit will bear its teeth to visitors, really big teeth belonging to sharks from the Indo-Pacific. In preparation for their arrival and other fish and sea turtles, artisans have been hard at work recreating that reef with fake coral.

PAUL VALIQUETTE: Here's a staghorn mold that we set up in port earlier. We can de-mold it, take the clamps off.

VIGELAND: Paul Valiquette heads the coral design team. He walks us through a warehouse where they've been hard at work sculpting, molding and painting fake coral that will bring the Blacktip Reef exhibit to life.

VALIQUETTE: And it just peels apart.

VIGELAND: Boy, that is taking you some effort to get that out of there.

VALIQUETTE: Yeah, this is a tough one.

(LAUGHTER)

VALIQUETTE: And then you would just clean up this flashing.

VIGELAND: It's something most aquarium visitors take for granted - the detailed coral reef structures that form a kaleidoscopic backdrop for the main attraction: sharks, rays, sea turtles. But most of that coral is not coral, which makes sense if you think about it.

Coral takes hundreds of years to grow into a reef, and curators can't exactly carve chunks out of natural reefs without destroying the ecosystem. So Valiquette and his team make it.

VALIQUETTE: The coral rock is made by creating a form and basically a big fiberglass shell. And on that goes texture, just some gravel and sand, and then it gets base painted with epoxy paint. All these different sponges and things are also polyester. They're sculpted on the piece.

VIGELAND: It's a deeply intricate process, but inspiration is just down the hall in giant tanks that look like they must have come from the set of a science fiction movie. Oh, my goodness. Sharks.

VALIQUETTE: Those are the guys. These are Blacktip Reef sharks headed to the exhibit.

VIGELAND: They're swimming around in what looks like a large aboveground pool. At the center of the shark pool, Valiquette points to a sample of his reef.

VALIQUETTE: What I'm testing for is usually the buoyancy - will it float or will it sink, or a color set up in the exhibit light and see the effect of the water depth on the color.

VIGELAND: The test coral also gives the reef sharks and the other marine animals a sense of the life that awaits them in the exhibit.

VALIQUETTE: When they go in the tank, we don't know how they're going to react to the reef. My concern is some of them - especially the turtle, I've seen turtles go in and just rip them all up. Hopefully, she won't do that.

(LAUGHTER)

VIGELAND: Less than a mile from the warehouse, we get a sneak peek at the aquarium's still-under-construction reef exhibit. Jack Cover is the general curator. He looks down over the tank as salt water slowly inches up over the fake reef.

JACK COVER: Yeah, what we're looking at here is a patch reef. And so we have a - different types of species of coral. And some are purple, some are bright yellow. You can see the brain corals down there which look like a brain. Some of them - or like the antlers on a deer, sponges, bright orange coral fan structures you're going to see deeper, they're all situated in the sort of the levels that they would be found on a re6al reef. It's kind of like the New York City of the ocean. It's a big structure. There's day sleepers, there's night sleepers. So there's activity going on all the time.

VIGELAND: So you have, what, the tenderloin and SoHo and Midtown, maybe?

COVER: Yeah. Well, there's a lot of little neighborhoods within this. Actually, you aren't too far off of that because we recreate the structure of these natural ecosystems.

VIGELAND: Given that this is not real coral, how do you convince the marine life in there that this is real when it's not?

COVER: Well, it's really the shapes they're really responding more to and they - the physical structure of it.

VALIQUETTE: So this is a coral cave. It's what you would find deep or, you know, undercover.

VIGELAND: Paul Valiquette takes us downstairs to what will be the underwater viewing area. We look through three large windows at the recreated coral reef. Pops of purple, red, orange, muted for now, in about four feet of murky blue water covering half the fabricated reef.

VALIQUETTE: So you have some things that are in the cave, and here's one of those staghorn thickets that'd be a great place where chromis and other small damsel fish would hang out. And then as you come around here to the big window, you see the coral rising and rising and rising up to the reef crest further back. The last window to the right here, these dense flats of small branching corals, there's a continuous network of caves at the base of that that abut right up to the window so you can see down inside. And there will be a lot of smaller animals hiding from the bigger predators up against the window. Well, that's our hope.

VIGELAND: A unique view of an Indo-Pacific reef and the corals that create them. Well, sort of. To see photos of our coral reef adventure at the National Aquarium, visit npr.org. Transcript provided by NPR, Copyright NPR.